In an atomic force microscope (AFM), a cantilever mounted with a sharp-tipped probe is used and the probe runs over a sample surface or traces with keeping a predetermined space from the sample surface in order to measure a displacement of the cantilever in a vertical direction due to an interatomic force between the sample and the probe, thereby evaluating the surface shape of the sample.
Here, an example of structure of an atomic force microscope (AFM) is specifically described. FIG. 6 is an explanatory diagram for describing the example of structure of the atomic force microscope.
In FIG. 6, the most basic structure of the atomic force microscope (AFM) is that a measuring part of the atomic force microscope (AFM) is placed above a vibration isolation table 101.
First, a cantilever 1 having a fine probe at its tip is vibrated by a vibrating part 102. Next, by using an optical microscope 103, the cantilever 1 is moved to a point above a place to be observed. Here, a Z stage 104 is gradually raised so as to bring the cantilever 1 closer to a sample 108 placed on a sample table 107.
When the cantilever 1 comes sufficiently close to the sample 108, an interatomic force (an attractive force or a repulsive force) acting between the probe at the tip of the cantilever 1 and the sample surface influences vibrations of the cantilever 1. When a distance between the probe and the sample is a predetermined distance (when they are at a standard position), the force influencing vibrations of the cantilever 1 is constant, and a deflection of the vibrating cantilever 1 is also constant.
Here, the atomic force microscope (AFM) detects from a deflection of the cantilever 1, and a reflection spot of a light beam from a laser 109 to irradiate the tip of the cantilever 1 is detected by a position detector 110. This optical detection system uses an “optical lever method”, and a subtle displacement of the cantilever 1 is magnified and projected onto the position detector 110. As the position detector 110, for example, a quadrant photodiode as shown in FIG. 6 is used. By computing a difference between respective detection signal amounts by a computation circuit, positional information is obtained.
That is, when the tip of the cantilever 1 is vertically displaced to cause a shift of the position of the reflection spot, a change occurs in the computation result of the difference between the detection signal amounts. Upon reception of this result, a differential amplifier 111 sends, to a power supply for driving piezoelectric element (on the Z stage 104), an output capable of feedback control over the distance between the probe and the sample so as to minimize a difference from a reference position, that is, to make the deflection amount of the cantilever constant.
A feedback circuit 112, for example, when the cantilever 1 is displaced upward, makes piezoelectric elements on the Z stage 104 shrink to cause the posture of the cantilever back to the original position.
In this manner, the atomic force microscope (AFM) scans the tested surface under feedback control that keeps the interatomic force acting between the probe and the sample constant, and, based on data obtained by distance-conversion of a Z-stage driving voltage at this time, an instruction is issued from a same computer 113 to an XY driving circuit 114 for moving the sample in X and Y directions to control an X stage 105 and a Y stage 106. In this manner, imaging is performed on the computer 113 as three-dimensional asperity information.
The spatial resolution of the atomic force microscope (AFM) depends on the radius of curvature of the probe at the tip of the cantilever 1, and the resolution is generally on the order of several nanometers.
According to what is called a magnetic force microscope (MFM), based on the atomic force microscope (AFM) inspection technology, the cantilever 1 being vapor-deposited with a magnetic material is used and is lifted up to a predetermined height from the surface of an object to be measured to measure a magnetic field occurring from the material surface above the object to be measured (about 10 to 30 nm thereabove).
An example of structure of this magnetic force microscope is shown in FIG. 7. FIG. 7 is an explanatory diagram for describing the example of structure of the magnetic force microscope (MFM).
In FIG. 7, a difference from the atomic force microscope (AFM) shown in FIG. 6 is that the cantilever 1 is lifted up to a predetermined height from the surface of an object to be measured to measure a magnetic field generated from the material surface above the object to be measured and also that the probe at the tip of the cantilever 1 is a magnetic-material-provided probe.
In general, it has been revealed by an experiment that, as with the atomic force microscope (AFM), the detectable spatial resolution of the magnetic field depends on the radius of curvature of the probe and others and, in practice, relates directly to the shape of the magnetic substance to be provided to the probe.
Here, an influence of a foreign substance when the magnetic force microscope (MFM) is used for measurement is described with reference to FIG. 8. FIG. 8 is an explanatory diagram for describing the influence of the foreign substance when the magnetic force microscope (MFM) is used for measurement.
A cantilever 1 of the magnetic force microscope (MFM) is at a predetermined height above the sample 108 to scan the sample 108. Therefore, as shown in FIG. 8, if a foreign substance 303 is present on the surface of the object to be measured from which a magnetic field to be measured 302 occurs, there is a high possibility that the probe is broken when the foreign substance collides with the probe at the tip of the cantilever 1 during high-speed scanning.
For this magnetic field microscope (MFM), a technology of providing a functional film such as a magnetic film on the probe at the tip of the cantilever is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2003-215020 (Patent Document 1).